diff --git a/PhysLean.lean b/PhysLean.lean
index 920ac6ec..9f1ced34 100644
--- a/PhysLean.lean
+++ b/PhysLean.lean
@@ -7,6 +7,7 @@ import PhysLean.ClassicalMechanics.HarmonicOscillator.ConfigurationSpace
 import PhysLean.ClassicalMechanics.HarmonicOscillator.Solution
 import PhysLean.ClassicalMechanics.Lagrangian.TotalDerivativeEquivalence
 import PhysLean.ClassicalMechanics.Mass.MassUnit
+import PhysLean.ClassicalMechanics.OrbitalMechanics.VisViva
 import PhysLean.ClassicalMechanics.Pendulum.CoplanarDoublePendulum
 import PhysLean.ClassicalMechanics.Pendulum.MiscellaneousPendulumPivotMotions
 import PhysLean.ClassicalMechanics.Pendulum.SlidingPendulum
diff --git a/PhysLean/ClassicalMechanics/OrbitalMechanics/VisViva.lean b/PhysLean/ClassicalMechanics/OrbitalMechanics/VisViva.lean
new file mode 100644
index 00000000..342fd26d
--- /dev/null
+++ b/PhysLean/ClassicalMechanics/OrbitalMechanics/VisViva.lean
@@ -0,0 +1,37 @@
+/-
+Copyright (c) 2026 Hannah Dawe. All rights reserved.
+Released under Apache 2.0 license as described in the file LICENSE.
+Authors: Hannah Dawe
+-/
+
+import Mathlib.Data.Real.Basic
+
+/-!
+# Circular Orbit Vis Viva
+Defines the orbital speed for a circular orbit (v^2 = G M / r).
+-/
+
+namespace ClassicalMechanics
+
+structure VisViva where
+  G : ℝ      -- gravitational constant
+  M : ℝ      -- central mass body
+  m : ℝ      -- orbiting mass body # for later usage
+
+namespace VisViva
+
+structure ConfigurationSpace where
+  r : ℝ      -- radius
+
+/-- Orbital speed for a circular orbit. -/
+def speedCircular (sys : VisViva) (cfg : ConfigurationSpace) : ℝ :=
+  Real.sqrt (sys.G * sys.M / cfg.r)
+
+/-- Lemma: the square of the circular orbit speed equals G M / r. -/
+lemma speedCircular_sq (sys : VisViva) (cfg : ConfigurationSpace) (hr : cfg.r > 0) :
+    (speedCircular sys cfg)^2 = sys.G * sys.M / cfg.r := by
+  simp [speedCircular, Real.sqrt_sq hr.le]
+
+end VisViva
+end ClassicalMechanics
+